Curious if the plugs were indeed bad the first/second time, or in hindsight were you chasing something else?
I am a firm believer in whatever the manufacturer tells you for scheduled maintenance cut it in half don’t go over 5k miles per oil change (I don’t buy into the cost of synthetics unless required).
Oil, filters, plugs are cheap.
I swapped my plugs at 50k and it was most certainly time. They gaps were out pretty bad especially the front two cylinders. A slight miss was also remedied. Especially spark plugs I don’t like the idea of something so finely threaded running heat cycles for 100k miles, especially true with aluminum block. I’ll spend a few bucks a plug every 50k.
I’ll swap coils at 75k most likely even though they aren’t scheduled maintenance item. Been hearing good things about the Accel coils in the F150s.
Just something the dealer that I purchased from offers on most of their newer used vehicles. I figured it would be under the caveat that all service would need to be performed in their service department. Fortunately not oil has to be changed at 5k and scheduled maintenance must be performed at an ASE certified shop, though every 5 years an inspection must be done by the dealer where I purchased (no charge).
Kind of sucks as I enjoy doing my own work but my local dealer passes out 24.95 oil change/tire roatation coupons like candy and scheduled maintenance is pretty spread out on this trucks.
I do all my other repairs and my half maintenance schedule. Small price to pay to have the motor, tranny, and 4wd components covered for life unlimited mileage.